1. Field of the Invention
The invention relates generally to preformed teeth for tooth roller cone rock bits.
2. Background Art
Drill bits used to drill wellbores through earth formations generally are made within one of two broad categories of bit structures. Drill bits in the first category are generally known as “fixed cutter” or “drag” bits, which usually include a bit body formed from steel or another high strength material and a plurality of cutting elements disposed at selected positions about the bit body. The cutting elements may be formed from any one or combination of hard or superhard materials, including, for example, natural or synthetic diamond, boron nitride, and tungsten carbide.
Drill bits of the second category are typically referred to as “roller cone” rock bits, which usually include a bit body having one or more roller cones rotatably mounted to the bit body. There are generally two “types” of roller cone cutting structures in the roller cone rock bits, the first being a tungsten carbide insert bit, (known as a TCI bit) and the second type being a tooth bit. In either case, the bit body is typically formed from steel or another high strength material. The roller cones are also typically formed from steel or other high strength material and include a plurality of cutting elements disposed at selected positions about the cones.
The cutting elements for TCI bits are commonly known as inserts or compacts and are typically made out of a hard material such as tungsten carbide with a cobalt binder and are typically press-fitted into holes drilled in the cones.
The process of which the method for making such inserts is commonly known in the art. The cutting elements for a tooth cone are commonly known as “teeth,” and are typically machined or formed into the cone. In typical applications, a layer of hardmetal is applied to the teeth to extend the wear life of the teeth.
Under normal drilling conditions, the relatively soft steel teeth of a milled-tooth cones are exposed to substantial abrasion and loading. This abrasion and loading can result in significant erosion and impact wear on the teeth. The wear on the teeth ultimately results in a reduction in the penetration rate of the drill bit and a shortened life of the drill bit.
A solution to the lack of wear resistance is to deposit a coating of wear-resistant material on the surfaces of the teeth. This process is sometimes referred to in the art as “hardfacing.”
Application of hardfacing to the base material from which the cones and drill bit are formed is known in the art. Typically, a hardfacing material is applied, such as by arc or gas welding, to the exterior surface of the teeth to improve the wear resistance of the teeth. The hardfacing material typically includes one or more metal carbides, which are bonded to the steel teeth by a metal alloy (“binder alloy”). In effect, the carbide particles are suspended in a matrix of metal forming a layer on the surface. The carbide particles give the hardfacing material hardness and wear resistance, while the matrix metal provides fracture toughness to the hardfacing.
Many factors affect the durability of a hardfacing composition in a particular application. These factors include the chemical composition and physical structure (size and shape) of the carbides, the chemical composition and microstructure of the matrix metal or alloy, and the relative proportions of the carbide materials to one another and to the matrix metal or alloy.
The metal carbide most commonly used in hardfacing is tungsten carbide.
Small amounts of tantalum carbide and titanium carbide may also be present in such material, although these other carbides are considered to be deleterious. It is quite common to refer to the material in the hardfacing merely as “carbide” without characterizing it as tungsten carbide. It should be understood that as used herein, “carbide” generally means tungsten carbide.
Many different types of tungsten carbides are known based on their different chemical compositions and physical structure. Three types of tungsten carbide commonly employed in hardfacing drill bits are: cast tungsten carbide, macro-crystalline tungsten carbide, and cemented tungsten carbide (also known as sintered tungsten carbide). The most common among these is possibly crushed cast carbide.
Tungsten forms two carbides, monotungsten carbide (WC) and ditungsten carbide (W2C). Tungsten carbide may also exist as a mixture of these two forms with any proportion between the two. Cast carbide is a eutectic mixture of the WC and W2C compounds, and as such the carbon content in cast carbide is substoichiometric, i.e., it has less carbon than the more desirable WC form of tungsten carbide. Cast carbide is prepared by freezing carbide from a molten state and crushing and comminuting the resultant particles to the desired particle size.
Macro-crystalline tungsten carbide is essentially stoichiometric WC in the form of single crystals. While most of the macro-crystalline tungsten carbide is in the form of single crystals, some bicrystals of WC are found in larger particles. Macro-crystalline WC is a desirable hardfacing material because of its toughness and stability.
The third type of tungsten carbide used in hardfacing is cemented tungsten carbide, also known as sintered tungsten carbide. Cemented tungsten carbide comprises small particles of tungsten carbide (e.g., 1 to 15 microns) bonded together with cobalt. Cemented tungsten carbide is made by mixing organic wax, tungsten carbide and cobalt powders, pressing the mixed powders to form a green compact, and “sintering” the composite at temperatures near the melting point of cobalt. The resulting dense cemented carbide can then be crushed and comminuted to form particles of cemented tungsten carbide for use in hardfacing.
In addition to these three types of commonly used carbides, carburized tungsten carbide may also be used to provide desired property. Other compositions for hardfacing are disclosed, for example in U.S. Pat. No. 4,836,307 issued to Keshavan et al., and U.S. Pat. No. RE 37,127 issued to Schader et al.
As mentioned above, conventional hardfacing usually comprises particles of tungsten carbide bonded to the steel teeth by a metal alloy. In effect, the carbide particles are suspended in a matrix of metal forming a layer on the surface. Most hardfacing on rock bits employs steel as the matrix, although other alloys may also be used. Such steel or other alloys will be generally referred to as a binder alloy. Hardfacing compositions are typically applied from tube rods, for example as disclosed in U.S. Pat. No. 5,250,355 issued to Newman et al.
A typical technique for applying hardfacing to the teeth on a rock bit is by oxyacetylene or atomic hydrogen welding. A welding “rod” or stick is typically formed of a tube of mild steel sheet enclosing a filler which mainly comprises carbide particles. The filler may also include deoxidizer for the steel, flux and a resin binder. The hardfacing is applied by melting an end of the rod on the face of the tooth. The steel tube melts to weld to the steel tooth and provide the matrix for the carbide particles. The deoxidizer alloys with the mild steel of the tube.
Although mild steel sheet is used when forming the tubes, the steel in the hardfacing on a finished a rock bit is a hard, wear resistant alloy steel. The conversion from a mild steel to the hard, wear resistant alloy steel occurs when the deoxidizers (which contain silicon and manganese) in the filler and tungsten, carbon, and possibly cobalt, from the tungsten carbide dissolve and mix with the steel during welding. There may also be some mixing with alloy steel from the teeth on the cone.
However, the above processes do not always produce satisfactory hardfacing coatings on milled teeth. Quality characteristics of a hardfacing coating are indicated, in part, by the thickness, uniformity, and coverage of the hardfacing coating on the tooth. The quality also is affected by the porosity of and the oxide and eta phase content in the coating. In a typical prior art process, the consistency of these characteristics varies from operator to operator and even from time to time for the same operator. Sometimes the quality of a hardfacing coating may differ significantly from one tooth to another on the same cone.
What is needed, therefore, are rock bits having consistent hardfacing layers, which can be used for a variety of applications, and methods for manufacturing the same.